1. Field of the Invention
This invention relates to elastic fluid turbines and more particularly to means for controlling the velocity of elastic fluid exhausting from such turbine within a predetermined range.
2. Description of the Prior Art
Steam turbines of relatively recent design have steam flow rates per area typically falling within the range of 12,000 pounds per hour per square foot to 18,000 pounds per hour per square foot with steam turbines employing saturated or wet steam having the relatively high flow rates per area of the previously described range. Expanding such enormous quantities of steam in a relatively reasonable number of separate flow paths require large cross-sectional area flows in the turbine's blade ducts with the last or exhaust stage blades of the turbine being correspondingly long. The exhaust stage or last row of blades of a typical large steam turbine converts about 6 percent of the total energy reduction of the steam into mechanical energy. Due to such high quality of energy conversion and large size of the exhaust stage turbine blades, extraordinary emphasis has been placed on the design of those blades.
Obtaining highest possible stage efficiencies and avoiding negative reactions on all turbine blades require axial velocities to be maintained within a specific range. Axial velocity of steam exiting a rotatable turbine blade is one of the most significant parameters for determining stage loading, probability of negative reaction, and probability of a turbine stage doing negative work. Last stage or exhaust blades in a turbine are the most difficult blades to optimally design since they are exposed to widely varying pressure ratios due to part load and overload operations. When exhaust pressures downstream from the exhaust stage vary, last stage blade optimization becomes even more difficult and often results in blades whose peak efficiencies may be rather low. Relatively small variations in exhaust pressure can have a substantial effect on turbine performance. The effect is especially pronounced when the turbine is operating at part load, during startup, or during shutdown where a change in back pressure of less than one inch of mercury for any given mass flow rate can cause the exhaust stage's mode of operation to change from zero work to choked flow or vice versa. The normal operation point for turbines is usually designed to fall between the two aforementioned extremes. Operation in the choked flow region would yield no additional turbine power output, but would increase the heat rate of the cycle whereas operation beyond the zero work region would cause consumption of, rather than production of, work generated by the remainder of the turbine blades. An additional disadvantage to operating beyond the zero work point is that the last stage would eventually experience the stall flutter phenomenon which can cause extraordinarily large blade vibrations. An additional reason for avoiding operation beyond the choke point is the discontinuous flow patterns which result upstream and downstream from the choke point. Such discontinuous and unsteady flow adds vectorially to any stimulating vibratory force on the blade caused by external forces.
Exhaust stage blading in actual service conditions starts choking at selected points along the blades and increases as the blade becomes fully choked. Attempts to provide uniform flow along the last stage turbine blades include extensive effort in providing a diffuser for the turbine's exhaust hood so that steam exiting the turbine's last stage blades at selected points does not "short-circuit" the diffuser and enter the condenser at high velocity.
Since the stall flutter phenomenon and unsteady, discontinuous flow occur beyond the zero work point and choke point respectively, any significant operation of the turbine beyond those points can adversely affect the life of the last stage turbine blades and thus the turbine itself. Precise control of the operating exhaust pressure would be highly desirable since such control would permit last stage turbine blade operation to be relegated to the flow region between the zero work point and choke flow point.